Recently, an organic electroluminescent element (hereinafter, referred to as “an organic EL element” as appropriate) using an organic substance has been regarded as promising as a large screen full-color display element of a solid-state luminescent type and a light source array for writing, making active research and development to be advanced.
The organic EL element is an all-solid-state element of a thin-film type which is configured by arranging an organic functional layer (single or multiple layer portions) including an organic luminescent substance of thickness of only about 0.1 μm between a pair of anode and cathode formed on a film. When the organic EL element like the above one is applied with a relatively low voltage of about 2 to 20 V, electrons are injected into the cathode of the organic compound layer and holes are injected into the anode thereof. It is known that when the electrodes and holes recombine in a luminescent layer, an energy level thereof returns from a conduction band to a valence band to discharge energy and light, thereby producing luminescence. This technique is expected to be applied in flat displays and illuminations.
Further, in an organic EL element utilizing recently discovered phosphorescence emission, luminescence efficiency of about four times as much can be achieved compared to that in the case of utilizing the conventional fluorescence emission. Therefore, as well as development of materials, research and development of layer configuration and electrodes for the luminescent element has also been carried out in a world-wide basis. In particular, as one of measures to prevent global warming, a study on application of the organic EL elements to illumination equipment is started to be made, and attempts have been actively made to improve performance and reduce costs to bring white luminescent panels into practice which can possibly replace the conventional illumination equipment. The white luminescent panel for illumination requires high efficiency and long lifetime; however, regarding the efficiency improvement, in particular, the luminescence efficiency thereof is still low compared to that of a fluorescent lamp and a white light-emitting diode.
Light extraction efficiency and element drive voltage are important factors that affect the luminescence efficiency of the organic EL element. In the current condition, light generated inside the organic EL element undergoes total reflection at the electrode and the like or is confined inside the element by being absorbed therein, resulting that most portion of the light is not effectively used. Specifically, in a case of a bottom-emission type organic luminescent element, in which metal is used for the cathode, it has been reported that light emitted in the luminescent layer becomes absorbed in the cathode by metal surface plasmon resonance. The amount of light extracted outside the element reduces as a result, and thus the luminescence efficiency of the element decreases.
As one means to suppress the above, it is effective to separate an emission position of the luminescent layer and the metal electrode; however, by thickening an electron transport layer to separate the luminescent layer and the cathode, electron transportability decreases and causes the voltage to increase and the luminescence efficiency to decrease. It is considered that these occur on the basis of a host material in the luminescent layer and carrier mobility of the electron transport layer, and at present, a material has not yet been discovered, which has a highly mobile electron transport property to suppress the voltage increase even if the layer is thickened.
As means to improve the transport property, an inorganic semiconductor can be used.
For example, in Patent Literature 1, the patent is disclosed in which inorganic nanoparticles are contained in an electric charge transport layer. It is known that carrier balance is adjusted by controlling the transportability in the electric charge transport layer by using the nanoparticles.